The present invention relates to a belt-drive mechanism to be used for driving mechanical devices such as a serial printer.
The belt-drive mechanism, which is composed of pulleys and a circular belt rounding around the pulleys, is used in various mechanical devices for transmitting rotary movement generated by a motor, by way of a driving pulley connected to the motor and the circular belt, to a driven mechanism connected to a driven pulley.
In the belt-drive mechanism, there is a problem of dispersion of length of the circular belt or distance between the pulleys, which may cause slips of the pulley because of lack of belt tension, or may make it difficult to set the circular belt to the pulleys because of lack of belt length. For dealing with this problem, various devices have been proposed.
FIG. 3 is a perspective view illustrating a belt-drive mechanism 1 disclosed in a Japanese patent application laid open as a Provisional Publication No. 227678/'91 for dealing with the above problem.
Referring to FIG. 3, the belt-drive mechanism 1 comprises a fixed pulley 4 which is set able to rotate around a shaft 3 provided at an end of a frame 2, a circular belt 5, and a driving pulley 7 coupled to a motor 6.
At the other end of the frame 2, there are provided a pulley hole 8 through which the driving pulley 7 is set, a fulcrum hole 10 through which a fulcrum screw 9 is set, and a long hole 12 having an arc shape through which a rotation screw 11 is set.
The belt-driving mechanism 1 comprises also an adjust plate 13, having an adjust plate pulley hole 14 through which the driving pulley 7 is set, and two through-holes 15 through which the fulcrum screw 9 and the rotation screw 11 are set.
With the fulcrum screw 9 and the rotation screw 11, which are both screwed into the motor 6 passing through bushes 16, the frame 2 and the adjust plate 13, the driving pulley 7 is fixed to the adjust plate 13, and set to the frame 2.
The driving pulley 7 fixed to the adjust plate 13 is able to swing around the fulcrum screw 9 against the frame 2, as the rotation screw 11 is set able to slide in the long hole 12.
For tensioning the adjust plate 13, a spring 19 is provided between a salient 17 of the adjust plate 13 and a heel piece 18 of the frame 2, in the belt-drive mechanism 1. The driving pulley 7 is tensioned to go away from the fixed pulley 4, as the adjust plate 13 is tensioned to swing counterclockwise, in FIG. 3, around the fulcrum screw 9.
Being thus assembled, the circular belt 5 of the above belt-driving mechanism 1 can be easily set around the driving pulley 7 and the fixed pulley 4 even when the size of the circular belt 5 is made a little shorter, since the distance between the driving pulley 7 and the fixed pulley 4 is adjustable by rotating the adjust plate 13, whereto the driving pulley 7 is fixed, against the frame 2.
Furthermore, as the driving pulley 7 is tensioned to go away from the fixed pulley 4 by the spring 19, an appropriate tension can be given to the circular belt 5 for preventing slips of the driving pulley 7, in the belt-drive mechanism 1.
However, a problem with the above belt-drive mechanism 1 is that the tension of the spring 19, and consequently, the tension of the circular belt 5 is degraded because of fatigue of the spring 19 according to time passage.
Another problem is the above belt-drive mechanism 1 is that a slip of the driving pulley 7 may occur when the motor 6 is started, for example, since the tension of circular belt 5 enlarged by large torque of the driving pulley 7 may extend the spring 19 and displace position of the driving pulley 7, resulting in tension fluctuation of the circular belt 5 itself.
Still another problem is the above belt-drive mechanism 1 is that the tension of the circular belt 5 can not be adjusted with a sufficient precision because the tension of the circular belt 7 is defined only by the tension of the spring 19 depending on extension length of the spring 19, which is to vary according to size dispersion of the circular belt 5, for example.